Lithium-ion technology is the leading technology in the field of rechargeable battery storage systems for portable electronics. Because of their high cell voltage, their superior energy density and power density and their exceedingly low self-discharge, lithium-ion batteries have high potential for these applications. Currently, lithium hexafluorophosphate (LiPF6) is being used as conductive salt in commercially available lithium-ion batteries. Lithium hexafluorophosphate has a relatively high conductivity, but has considerable disadvantages because of low thermal and chemical stability and its hydrolysis sensitivity. Thus, it is known that LiPF6 reacts with traces of water and other protic compounds such as alcohols, which are not entirely avoidable in lithium batteries and occur in solvents, for example, in the ppm range. This reaction is accelerated by moderately elevated temperatures. This results in a rapid loss of cell capacity, which is manifested in a shortened lifetime.
There are therefore intensive efforts to develop alternative lithium salts which can replace LiPF6 as conductive salt. The lithium salts that have been developed in the last few years are frequently complex boron- or phosphorus-containing anions having nonaromatic chelating agents such as oxalate, for example lithium bis(oxalato)borate (LiBOB), which is disclosed in DE 198 29 030 C1. It is disadvantageous, however, that bis(oxalato)borate has only a low solubility in the carbonates that are typically used as solvents in electrolytes. Moreover, LiBOB-based electrolytes have a lower conductivity compared to LiPF6, especially at low temperatures, and a higher viscosity.
In spite of a multitude of salts and solvents, no suitable substitute has been found as yet for LiPF6 as conductive salt in carbonate mixtures. There is therefore a need for alternative lithium salts for use in lithium-ion batteries.